On a holiday with friends I once spent some time on a beach trying to demonstrate a computer could be built with sandcastles and tennis balls. IIRC, we managed

On a holiday with friends I once spent some time on a beach trying to demonstrate a computer could be built with sandcastles and tennis balls. IIRC, we managed to get AND, OR, NOT, XOR working and I set about building a half adder. It needed a lot of beach space, since it was a fairly shallow slope and you needed the balls to get up enough speed to work the logic. The end result was, unfortunately, that the sea started washing away my design before I quite managed to debug it. Ultimately I'd have needed to find a way to build a bigger chunk of logic and keep the balls circulating around it - didn't get that far but felt ready to do so if a post-apocalyptic scenario called for it.